The invention relates to a method of arc spraying, and to an apparatus for arc spraying.
Arc spraying, frequently also more accurately called arc wire thermal spraying in the technical literature, is a conventional technology for the manufacture of surface coatings on workpieces which should be protected, for example, against mechanical wear, corrosion or against chemical or thermal strains.
With arc spraying, a wire-like or tube-like spray medium in the form of two spray wires is melted in an electrical arc using a spray gun and sprayed onto a workpiece surface by an atomizing gas, e.g. by nitrogen, a noble gas or simply by air, under a pre-settable pressure. In this connection, the arc is initiated between the two ends of the spray wires by application of an electrical voltage and contact ignition. This is different from so-called “flame spraying”, a method in which the thermal energy to melt the spray wire is applied by a combustible gas/oxygen flame, whereas in arc spraying, the electrical energy released in the arc supplies the required thermal energy to melt the spray wires.
Since the material of the spray wires is converted into the melt in the region of the arc and is sprayed onto the surface of the material, the spray wire must be continuously fed from a wire store by a wire feed. Depending on the application, stationary spray guns are known which are frequently used in automated operation for the processing of large series, but also relatively small hand spray guns can be used which allow a more flexible employment. The device for the wire feed can here be either installed in the spray gun itself or, however, be effected by a wire feed unit lying outside the spray gun.
In this connection, the properties of the sprayed layers can be directly influenced by different parameters such as the wire diameter, the material of the spray wire, the speed of the wire feed, the electrical voltage for the generation of the arc or the electrical current for the maintaining of the arc, the selection of the atomizing gas and its working pressure or the spray distance. That is, as a rule, a change of one or more of these parameters (or also of other parameters not named here) will result in layers with different properties and quality. Since the previously named parameters can generally be easily influenced by a selection of the materials or by electronic control and/or regulation devices, the arc spraying is characterized by high flexibility. For example, among other things, the spray droplet size, or the kinetic energy of the spray droplet, can easily be set automatically, or also during an on-going coating procedure, depending on the demand. Moreover, the spray procedure itself can take place under a normal ambient atmosphere, in a vacuum chamber or in an insert gas environment. A wide selection of the most varied workpieces with the most varied demands on properties and quality can thereby be provided with protective surfaces using the method of arc spraying.
A disadvantage of the known methods of arc spraying can be seen in the fact that quite specific demands are to be made on the wire materials which come into question for the manufacture of the spray wires. For instance the spray wires must have a sufficient ductility, that is, a sufficiently high deformability and/or elasticity and a sufficiently high conductivity. If components with electrically poor conductivity or no conductivity or relatively hard, that is, less ductile, components such as ceramic materials should also be introduced into a layer to be sprayed, so-called filler wires must be made use of. Filler wires are understood to be spray wires with which additional components, usually in the form of discrete particles, are included in the base material of the spray wire, which do not melt or only start to melt in the arc, and are also installed in the surface layer sprayed on. In this connection, the particles are very limited in their volumes and can partly already be changed in their structure by the melting procedure of the spray wire, which frequently results in undesired modifications in the layer. The volume portion of the particles in the melt can also practically not be changed in a controlled manner since it is fixedly predetermined by the distribution of the particles in the filler wire. The size, shape and type of the particles can also only be changed by replacing the filler wire and thus not be changed during the spray procedure itself. In addition, electrically non-conductive particles in the spray wire can disturb the stability of the arc and thus influence the quality of the sprayed surface layer in a substantially negative manner. Furthermore, the manufacture of filler wires is correspondingly complex and expensive in comparison with usual spray wires.